This invention relates to heat sinks and, more particularly, to a multilayer composite metallic sheet having a predetermined coefficient of thermal expansion for use as a heat sink and mounting arrangement for high power ceramic thick film hybrid packages.
It has been necessary in the past to bond metallic heat sinks to ceramic substrates. Clearly, to avoid the problem of fracturing during subsequent thermal processing, it has been necessary to match the coefficient of thermal expansion of the ceramic substrate with that of the metallic heat sink which is bonded thereto. A substance known as Kovar has been developed which has expansion characteristics similar to those of glass and has been used as a heat sink material for ceramic substrates. However, the ceramic substrates have generally been rather small (in order of 1".times. 1") and therefore, the poor heat dissipating characteristics of Kovar did not represent a major problem since the small ceramic hybrids were low power devices and therefore did not generate a large amount of heat. The trend towards large scale integration (LSI) wherein entire micro-processors are placed on a single ceramic substrate has made it necessary to resort to larger ceramic packages. Therefore, it is necessary that the heat sink, which is bonded to the package, has a coefficient of thermal expansion which substantially matches that of the ceramic to avoid fracturing and superior heat dissipating properties in order to accommodate the higher power hybrid. Kovar has been proved to be unsuitable for the larger, higher power devices.